Geology Reference
In-Depth Information
(a)
(b)
(c)
Figure 9.18. Possible tectonic events accompanying a mantle overturn. Hot mate-
rial from the lower mantle (stippled) replaces cooler material in the upper mantle
(grey). As a result, thicker oceanic crust is formed, and flood-basalt-like eruptions
occur on continents. The thickened oceanic crust blocks subduction in (c). From
Davies [175]. Copyright Elsevier Science. Reprinted with permission.
their record is left in ocean basins and because their basaltic products are readily
weathered away.
One kind of candidate for inferring past plume activity may be large igneous
provinces, inferred to come from mantle plumes [170]. Indeed the possibility that
Archaean greenstone belts are the products of mantle plumes was an important
stimulus to developing the modern theory of plume heads [167, 168]. Another
signature of a plume head may be radiating dyke swarms, inferred to be the feeder
dykes of flood basalt provinces [169, 170].
The apparent episodicity of the continental record requires something beyond the
smoothly evolving models of Figures 9.1, 9.2 and 9.6. The basalt barrier mechanism
of Section 9.4.3 certainly induces episodic behaviour of the right general kind,
especially in having overturns that would produce major magmatic and tectonic
episodes. The material rising from the lower mantle is not only 200-300
◦
C hotter
than the upper mantle, but also more fertile. The volume of magma potentially
involved, as indicated by the spikes in the thickness of oceanic crust, is perhaps
200 times more than in a flood basalt eruption. Figure 9.18 sketches how such an
